Scissors lift mechanisms in general are known to the art. The principal purpose of such mechanisms is to provide a safe and efficient means for supporting a working platform at any desired elevation. The scissors lift mechanisms of the prior art are predicated on the well-known "lazy tong" principle, and each comprises a pair of vertically extensible scissors linkages mounted on a frame in laterally spaced, parallel relationship, and a working platform mounted on top of the linkages.
Each of the scissors linkages of the prior art lift mechanisms comprises pairs of arms pivotally connected to one another at their ends and at their centers. The lowermost pairs of arms of the linkages are pivotally mounted at one end to the frame, and they are slidably mounted on the frame at their other ends. An hydraulic drive, or lift, cylinder mechanism is pivotally mounted to the frame in the prior art mechanisms, and it is coupled to a cross-bar extending between the lowermost pairs of arms of the linkages. The hydraulic mechanism serves to turn the arms of the lower most pair about their pivotal axis to extend or retract the linkages and thereby to raise or lower the platform.
The frame is mounted on wheels in most of the prior art lift mechanisms, and an appropriate drive motor is provided to move the lift from one location to another. A control box is usually mounted on the working platform to enable the worker to control the elevation of the platform, and also to move the lift from one location to another, thereby obviating the need for a separate driver.
Each of the scissors linkages of the prior art lift mechanisms comprise pairs of arms pivotally connected to one another at their ends and at their centers. The lowermost pairs of arms of the linkages are pivotally mounted at one end to the frame, and they are slidably mounted on the frame at their other end. It is usual in the prior art scissors lift mechanisms to provide an hydraulic drive cylinder mechanism which is pivotally mounted to the frame, and which is coupleted to a cross-bar extending between the lowermost pairs of arms of the linkages. The hydraulic lift mechanism serves to turn the arms of the lowermost pair about their pivotal axis to extend or retract the linkages and thereby to raise or lower the platform.
A disadvantage in the prior art hydraulic drive is the fact that as the lift mechanism is initially elevated from its lowermost position, the hydraulic cylinder/piston unit of the prior art hydraulic mechanism is positioned almost horizontal, and it must exert an excessively high thrust on the mechanism to turn the lowermost arms and to start the vertical extension of the linkages.
Then, as the prior art lift is extended more and more in a vertical direction, the hydraulic lift unit pivots to an upright position, and it requires less and less thrust to move the load. This results in the need for an excessively large hydraulic lift unit in the prior art scissors lift in order to be effective to move the linkages from their retracted to their fully extended position, and it often leads to the requirement for auxiliary hydraulic lift mechanisms.
In addition to the above, the lift mechanism of the prior art are designed so that the arms do not necessarily open uniformly due to the fact that the arms tend to deflect at unsupported locations thereon. In essence, when examining the pivot points connecting a pair of arms in each of a pair of transversely spaced apart scissors linkages, the pivot points in the spaced apart lowermost arms will have a variable difference with respect to the spacing between the pivot points in the uppermost arms during the initial opening. This problem results from the inelastic instability which is inherent in a beam of the type constituting an arm in a scissors linkage.
There have been many attempts to overcome the problems created by the need for excessively large hydraulic lift units, and to covercome the problems created by the inelastic instability in the scissors lift arms with attempts to employ some form of a somewhat vertically disposed hydraulic lift unit. Thus, one such attempt has been described in the U.S. Pat. No. 3,259,369 to Gridley. However, such prior art attempts have usually resulted in excessive structure in the scissors lift unit in order to support the hydraulic unit.
Moreover, in most constructions, these hydraulic units had at least one end thereof directly connected to the pivot point or to a member which was co-parallel in space with a pivot point connecting two corresponding arms of two transversely spaced apart scissors linkages. This structure tended to create some inherent instability and also required a greater amount of opening force when compared to offsetting the hydraulic drive units from the pivot points.
One of the primary problems of the lift devices of the prior art is that the linkages forming part of the scissors lift were not constructed so that they could be collapsed to a minimum height. Moreover, and by virtue of this fact, the arms in each of the linkages were not supported so as to relieve stress at the pivotal points of the linkage. Thus, the arms alone did not support the total weight of the platform when the linkages were in the collapsed position, and, consequently, a considerable amount of the stress was imposed at the pivotal points connecting the various arms of the linkages. This construction not only resulted in a substantially reduced overall life of the lift mechanism, but also created greater necessity for repairs and maintenance.
There has been at least one attempt to employ brackets at the ends of the arms which serve to offset the pivotal axes of the arms away from their respective longitudinal axes. This construction was designed to permit the linkages to be collapsed in a manner where the arms lie directly upon the next lowermost arm. Nevertheless, this construction, while somewhat effective, really did not provide its maximum utility. One of the principal problems with respect to the brackets which offset the pivotal axes of the arms is that in the prior art lift mechanisms, the hydraulic cylinder units were not essentially located in order to provide vertically extensible thrust and were not necessarily designed to provide uniform load transfer to each of the arms in the scissors lift mechanism. Consequently, in such prior art devices, the principle of offsetting the pivotal axes of the arms did not assume the full advantage of this technique.
There have been several other attempts in the prior art to provide ear-like structures on the ends of the arms forming part of the scissors linkage in order to attempt to create a minimum collapsed condition of the linkages. One such attempt has been described in U.S. Pat. No. 3,672,104 to Luckey. However, in this attempt, as well as in each of these other attempts, the prior art did not recognize that the employment of ears on the ends of the arms forming part of the linkages which offset the pivotal axis of the arms away from their respective longitudinal axis was most effectively important in conjunction with hydraulic lift units which provided an essentially vertical thrust and which thereby permitted the hydraulic unit to create movement from the fully compact position to a fully extended position with a minimum of capacity and in such manner that the thrust exerted thereby was essentially invariable to move the load through all positions of the linkages.
It is, therefore, the primary object of the present invention to provide a lift in which the hydraulic mechanism is capable of performing a desired function with less thrust and more capacity requirement than any prior art mechanism and on a more economical and safer basis and permits the linkages of the lift to be collapsed to a minimum height.
It is another object of the present invention to provide a lift of the type stated which provides uniform load transfer to each of the arms in the scissors lift mechanism forming part of the lift.
It is a further object of the present invention to provide a lift of the type stated which overcomes the inelastic instability which otherwise results in beam deflection in prior art types of lift devices.
It is an additional object of the present invention to provide a scissors lift mechanism of the type stated cooperating with hydraulic power unit and which are capable of being used in a wide variety of devices.
It is another salient object of the present invention to provide a scissors lift of the type stated which permits the linkages to be collapsed to a minimum height by use of ear-shaped brackets at the ends of the arms of the linkages.
With the above and other objects in view, my invention resides in the novel features and form, construction, arrangement and combination of parts presently described and pointed out in the claims.